In contrast to so-called finger sensors, planar sensor elements as they are used to determine the oxygen concentration in the exhaust gas of internal combustion engines in so-called lambda sensors, for example, are distinguished in that all functional layers, such as Nernst cell and reference-gas channel, are implemented as planar, superposed surfaces. A planar sensor element designed for a broadband lambda sensor is described in German Patent Application No. DE 199 41 051. A planar sensor element used for a jump sensor or lambda=1 sensor is to be found in “Wiedenmann, Hötzel, Neumann, Riegel, Weyl ZrO2 Lambda Sensors For The Mixture Control In The Motor Vehicle, B. G. Teubner Stuttgart, 1995, pages 383 and 384”.
In order to comply with higher demands regarding the precision of sensors as a result of more stringent emission standards for motor vehicles, the sensor elements are predominantly designed to include an integrated heater; unheated sensor elements, on the other hand, are no longer of great importance in the motor-vehicle sector.